Translation termination is the final stage of protein synthesis. It includes at least two essential functions, stop codon recognition and polypeptide chain release. In eukaryotic organisms, the class I release factor eRF1 recognizes each of the three termination codons (UAA, UAG, and UGA) and mediates release of the nascent polypeptide chain. The class II release factor eRF3 assists the termination process in a GTP-dependent manner. The long-term goal of this project is to better understand the process of translation termination so therapeutic strategies aimed at the suppression of disease-causing premature stop mutations can be developed. The eRF1 protein contains three discrete domains. A consideration of structural and genetic data led to the proposal that domain 1 mediates stop codon recognition;domain 2 interacts with the peptidyl transferase center of the ribosome to facilitate polypeptide chain release;and domain 3 mediates the interaction between eRF1 with eRF3. Competing models argue that either the TASNIKS motif or the YCF motif in domain 1 is critical for stop codon recognition. The first aim of this proposal will identify key residues of eRF1 domain 1 involved in stop codon recognition to test the relative merits of these competing models. The yeast SUP45 gene encodes eRF1. We recently discovered that the half-life of the SUP45 mRNA is regulated by the efficiency of the termination process. This mechanism leads to an increase in the eRF1 protein level when termination is compromised. The second aim of this proposal will test this model and explore how this novel regulatory mechanism controls SUP45 mRNA and eRF1 protein levels. We recently found that the previously uncharacterized protein Tpa1p influences the efficiency of translation termination, mRNA poly(A) tail length, and mRNA half-life in yeast cells. This led us to propose a model in which Tpa1p couples translation termination to the deadenylation of cellular mRNAs. The third aim of this proposal will test various aspects of this model so we can better understand the important interplay between translation termination and mRNA stability.